Repair of DNA damage caused by various mutagenic chemicals, radiation, and the environmental agents is fundamental to a cell's survival. Failure of DNA repair often results in genetic instability. DNA double strand break (DSB) is one of the most lethal forms of DNA damage. All eukaryotes thus have multiple mechanisms to deal with DSBs. The importance of understanding eukaryotic DSB repair pathways is highlighted by the existence of several human diseases that are marked by immune dysfunction and predisposition of the affected individuals to cancer. These diseases are the consequences of mutations in DNA repair genes. The long-term goal of our laboratory is to define this casual linkage between the genetic instability and DSB repair defect. To accomplish this goal, we need to understand the detailed molecular mechanisms of DSB repair. This proposal will identify and characterize the genetic components involved in non-homologous end joining (NHEJ), major DSB repair pathway in all eukaryotes, in a genetically tractable yeast model system. Elucidation of the physical and genetic interactions among NHEJ components will be achieved by a combination of genetic and biochemical approaches. Together, our studies should yield the mechanistic information covering the molecular mechanisms of this critical DNA repair process. Furthermore, since DSB repair by NHEJ is remarkably conserved from yeast to humans, these studies will help to dissect the similar pathways in humans.